Ethylene acid-catalyzed dehydration reactions

According to Olah s investigations the conversion of methyl alcohol over bifunctional acidic-basic catalyst after initial acid-catalyzed dehydration to dimethyl ether involves oxonium ion formation catalyzed also by the acid functionality of the catalyst. This is followed by basic site catalyzed deprotonation to a reactive surface-bound oxonium ylide, which is then immediately methylated by excess methyl alcohol or dimethyl ether leading to the crucial - 2 conversion step. The ethyl methyl oxonium ion formed subsequently eliminates ethylene. All other hydrocarbons are derived from ethylene by known oligomerization-fragmentation chemistry. Propylene is formed via a cyclopropane intermediate. The overall reaction sequence is depicted in Scheme 19. 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

The reaction is catalyzed by all but the weakest acids. In the dehydration of ethanol over heterogeneous catalysts, such as alumina (342—346), ether is the main product below 260°C at higher temperatures both ether and ethylene are produced. Other catalysts used include silica—alumina (347,348), copper sulfate, tin chloride, manganous chloride, aluminum chloride, chrome alum, and chromium sulfate (349,350). 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

Many of the trial results have indicated that the coexistence of an acid and alkaline center is conducive to dehydration of ethanol to ethylene. Hassan suggested the catalytic mechanism of ethanol dehydration catalyzed by solid acid or alkaline catalysts in 1982 (Abd El-Salaam and Hassan 1982). He claimed the acid and alkaline center of the catalyst cooperated in the process of ethanol dehydration. Although the reaction was mainly catalyzed by the acid center, the existence of a modest alkaline center could promote this reaction. Hassan considered ethanol was absorbed in the acid and alkaline center on the surface of the catalyst and then formed adsorption-state compounds, which could further dehydrate to ethylene and release the acid and alkaline center. The mechanism of dehydration of ethanol to ethylene catalyzed by activated alumina suggested by Cosimo et al. (1998) is shown Fig. 4. 

---